Conventional carburetors for internal fuel combustion engines are known to have a fuel and air mixing passage for delivering a controlled ratio of fuel-and-air to the combustion chamber of a running two or four stroke engine. The mixing passage is defined by a body of the carburetor and has a venturi disposed between an upstream region and a downstream region of the passage. Generally controlling or limiting the amount of air flowing through the venturi is a choke valve of a butterfly-type disposed within the upstream region of the passage. Generally controlling the amount of fuel-and-air mixture fed to the combustion chamber of a running engine is a throttle valve, also of a butterfly-type, which is disposed within the downstream region of the passage. As the throttle valve rotates from a substantially closed position to a wide open throttle position and the choke valve is open, the engine rpm will generally increase from idle to maximum or full power. At wide open throttle, a vacuum induced at the venturi increases with the increased air flow demand of the engine. This causes an increase in fuel flow typically from a near atmospheric fuel supply chamber, through a fuel feed passage and a fuel orifice disposed at a radially most inward portion of the venturi.
The ratio of fuel-to-air of a running engine is generally less than the ratio necessary to reliably start a cold engine. The choke valve is primarily necessary to adjust the fuel-to-air ratio by controlling the air flow rate through the upstream region of the mixing passage. Prior to starting of a cold engine, the user must first manually place the choke valve in a substantially closed or “choke-on” position. The air flow is thus limited and a rich mixture of fuel-and-air flows through an intake manifold and to the combustion chamber of the engine via the pulsating vacuum induced by the reciprocating piston(s) of the engine.
Once the engine has started, the user must remember to manual place the choke valve in an open or “choke-off” position to lean-out the fuel-and-air mixture to achieve smooth running of the engine. If the user does not timely remember to manually place the choke valve in an open or “choke-off” position after start-up, and during idle conditions, the engine may stall on an overly rich mixture of fuel-and-air, or, a black smoke will be emitted from the exhaust, indicative of an unwanted increase in hydro-carbon emissions. Moreover, if the user attempts to increase rpm's of the idling engine with the choke valve substantially closed, the air demands of the engine will not be met and the engine will stall on an excessively rich mixture of fuel-and-air.
The butterfly-type choke valve has a rotating shaft which traverses the mixing passage and extends through the body of the carburetor. A pivoting plate of the choke valve located within the upstream region of the mixing passage is secured rigidly to the rotating shaft, and when closed conforms in shape to the contours of the mixing passage. Usually the choke valve is retained in its closed and open positions by a detent arrangement.
For initial start up of the engine, the choke valve is manually moved to its closed position. Once the engine is running, the user typically must manually move the choke valve to its open position to allow an increase air flow for higher engine speeds and to prevent the engine from stalling due to an overly rich mixture of fuel and air.